This requires your host to have: #ServerAlias *.domain.com in the .htaccess file of in the config of Apache otherwise Apache just redirects to a domain name.

<?php

// Whether or not to output debug code
//DEFINE("DEBUG", 1);
DEFINE("DEBUG", 0);

// An array of actual subdomains on the server
$real_subdomains = array('subdomain1','subdomain2');

// The domain info for this site
$domain_name = 'domain';
$domain_tld = '.com';

// Set a variable to hold the host info
$host_info = '';

// Check to see if 'register_globals' is on
if(ini_set('register_globals' == 1))
	{
	$host_info = $HTTP_HOST;
	}
else
	{
	$host_info = $_SERVER['HTTP_HOST'];
	}

// Break up the pieces of the host info
$parts = explode(".", $host_info);

// Pull the 'subdomain' for the url
$subdomain = $parts[0];

(DEBUG) ? print "Host Info: " . $host_info : '';

// Check that the subdomain isn't 'www' or the domain name
if(($subdomain != "www") && ($subdomain != $domain_name))
	{
	// Is the subdomain a real domain?
	if(in_array($subdomain, $real_subdomains))
	{
	// Redirect and exit
	header("Location: http://www." . $domain_name . $domain_tld . "/" . $subdomain . "/");
	exit();
	}
	else
	{
	header("Location: http://www." . $domain_name . $domain_tld . "/");
	}
	}

?>

The above code has been commented for easy use. This will work if "register_globals" is on or off.